The present invention relates to chain saw guide bars and, in particular, to a replaceable nose member for a chain saw guide bar.
It is well known in the prior art to provide chain saw guide bars with separate nose members, because the nose member preferably has a considerably different structure as compared to the body of the guide bar. That is, a nose member is more dependable if made from a number of thinner plates which have smooth contact surfaces bearing against a nose sprocket, or against a nose wheel and its bearing parts, whereas the body of the guide bar is stronger and stiffer if it is made from one thick plate rather than a number of thinner plates welded, riveted or laminated together.
It is also an advantage to be able to replace a nose member since the nose sprocket or its bearing is worn out or damaged in service more often than the guide bar body. The nose member is therefore commonly made as a separate item which can be slid into place and fastened to the guide bar body.
The saw chain can be fractured in service if it encounters nails or other hard objects, and may then fall off. To avoid losing the nose member in case the chain falls off, it has been traditional to attach the nose member to the guide bar body by rivets as disclosed in U.S. Pat. No. 596,802 and later patents. When the nose member is to be replaced, the rivets have to be removed by drilling, and other rivets have to be set to attach a new nose member. Consequently, exchange of such a nose member cannot be done in the field. A tight attachment will also reduce vibrations and noise, and preserve the continuity of the peripheral groove.
Alternative fastening methods have been suggested, such as attaching the nose member by a rotatable claw or other reusable element as in U.S. Pat. No. 4,956,918. Holding the nose member in place by a tight press fit was suggested in U.S. Pat. No. 2,693,206. Press fit as well as screws have found very little practical application, since the extreme vibrations tend to loosen such connections and make them unreliable. An elastic wedge-shaped center plate was suggested in CA 493,111, but will be extremely sensitive to vibrations, since the wedge action will make the force needed to release the nose member much smaller than the force needed to mount it.
It has also been proposed to secure a nose member by a snap-in connection which includes hook-shaped ears that flex elastically in a direction within the plane of the guide bar, i.e., perpendicular to a thickness direction of the guide bar, e.g., see U.S. Pat. No. 4,060,895. However, the ears must inherently be thin and thus may not be sufficiently sturdy to withstand out-of-plane deflections of the nose member. Also, out-of-plane deflections of the nose member could damage the ears.
The present invention involves attaching a replaceable nose member to a guide bar body by a snap-in coupling, requiring no parts such as fasteners, that could be lost or loosened by vibration, while allowing replacement of a nose member in the field using no tools more complicated than a screwdriver. The present invention relates to a chainsaw guide bar which comprises a guide bar body and a replaceable nose member. The nose member has two rearward tongues spaced apart in a thickness direction of the nose member. The body has a web which fits between the tongues. The nose member is held in place by an elastic snap-in coupling. The snap-in coupling comprises spring portions defined by indentations formed in respective ones of the tongues and projecting toward the web. The web includes a cut-out arrangement for receiving the indentations. The spring portions are elastically flexible in a thickness direction of the guide bar for enabling the indentations to be slidable into place and out of place without adding or removing any fastening element.
Preferably, there is a plurality of indentations formed in each of the tongues on opposite sides of the center axis. The cut-out arrangement preferably comprises through-holes formed in the web, and positioned to receive the indentations.
A space is preferably formed between the nose member and the bar body for receiving a prying tool.
The invention also pertains to a nose member whose tongues include dimples formed on opposite sides of a center axis of the nose member.